PVC vs Polyethylene: Monomers, Repeat Units and Uses

A flexible electrical cable sheath and a rigid plumbing pipe can both be made from PVC. A soft shopping bag and a rigid water pipe can both be made from polyethylene. So how do you tell these two polymers apart, and why can materials made from similar carbon chains have such different properties?

The answer starts with their monomers and repeat units. PVC is made from chloroethene, while polyethylene (PE) is made from ethene. The key structural difference is that PVC contains chlorine atoms attached to its carbon backbone, whereas polyethylene contains only carbon and hydrogen.

Before looking at the structures, make a prediction: does the presence of chlorine automatically make PVC rigid? Not quite. The chlorine changes the interactions between polymer chains, but additives can also change how the finished material behaves.

01PVC vs polyethylene: the monomers and repeat units

Let’s establish the most important difference first.

Both polymers are produced through addition polymerisation. Their carbon-carbon double bonds open, allowing the monomers to join into long chains.

The resulting structures are:

Polyethylene:

\[ n\ce{CH2=CH2} \longrightarrow \ce{[-CH2-CH2-]_{n}} \]

PVC:

\[ n\ce{CH2=CHCl} \longrightarrow \ce{[-CH2-CH(Cl)-]_{n}} \]

Here, \(n\) represents the number of repeating units in the polymer chain.

Notice that both polymers have a backbone consisting of carbon-carbon single bonds. The difference is what is attached to those carbon atoms.

• In polyethylene, each backbone carbon in the repeat unit is bonded to two hydrogen atoms.
• In PVC, one backbone carbon is bonded to one hydrogen atom and one chlorine atom.

The chlorine atom remains attached to the carbon backbone throughout polymerisation. It does not disappear, and it does not become part of the main carbon chain.

The structural difference worth remembering

If you are given a polymer structure in an examination, look for the chlorine atom.

A repeating \(\ce{-CH2-CH2-}\) unit identifies polyethylene.

A repeating \(\ce{-CH2-CH(Cl)-}\) unit identifies PVC.

This is more reliable than trying to identify a polymer from its appearance or its intended use.

02Why does the chlorine atom matter?

The chlorine atom does more than increase the mass of PVC.

It changes the way neighbouring polymer chains interact.

Start with polyethylene. Its backbone contains carbon-carbon and carbon-hydrogen bonds. The polymer is largely non-polar, so the intermolecular attractions between neighbouring chains are mainly London dispersion forces.

PVC is different. Its carbon-chlorine bonds are polar because chlorine attracts the shared bonding electrons more strongly than carbon.

This produces regions of partial positive and partial negative charge within the polymer structure.

Neighbouring PVC chains can therefore experience dipole-dipole attractions as well as London dispersion forces.

The chlorine atom also makes the PVC repeat unit considerably heavier.

For polyethylene, the repeat unit has a molar mass of approximately \(28.0\text{ g mol}^{-1}\).

For PVC, it is approximately \(62.5\text{ g mol}^{-1}\).

That is more than twice the mass per mole of repeat units.

However, don’t jump to the conclusion that stronger intermolecular attractions automatically mean a rigid material. The movement of polymer chains, their arrangement, and the presence of additives also matter.

03Polyethylene: why LDPE and HDPE behave differently

Here is a detail that often catches students out.

Polyethylene is not just one material with one fixed set of properties. Two important commercial forms are:

• Low-density polyethylene (LDPE).
• High-density polyethylene (HDPE).

Both are made from ethene, and both have the same basic repeat unit:

\[ \ce{[-CH2-CH2-]_{n}} \]

So why are their properties different?

The answer is the arrangement of their polymer chains.

Low-density polyethylene (LDPE)

LDPE contains more branching along its carbon chains.

These branches make it harder for neighbouring chains to pack closely together. This generally reduces the degree of crystallinity and produces a less dense, more flexible material.

LDPE is commonly used for:

• Plastic shopping bags.
• Flexible packaging.
• Plastic films.
• Squeeze bottles.

High-density polyethylene (HDPE)

HDPE has much less branching and contains relatively linear polymer chains.

These chains can pack more closely together, producing a higher density and generally greater stiffness.

HDPE is commonly used for:

• Milk and detergent bottles.
• Rigid containers.
• Water pipes.
• Plastic storage bins.

A useful way to picture this is to imagine arranging long pieces of string in a box.

Straight pieces can lie relatively close together. Pieces with lots of branches get in the way, leaving more space between neighbouring strands.

Real polymer chains are much more complicated: they fold, tangle, and form ordered and disordered regions. The string comparison simply helps explain why branching affects packing.

The important distinction

LDPE and HDPE have different chain architectures, not different basic repeat units.

This is a different structural issue from the chlorine substituent in PVC.

• PVC has chlorine atoms attached to its carbon backbone.
• LDPE has carbon-chain branches.
• HDPE has relatively linear carbon chains.

Don’t confuse a chlorine substituent with a carbon-chain branch.

04PVC: why can it be rigid or flexible?

You might now expect PVC to behave as one consistently rigid material.

But look at two familiar applications:

• Unplasticised PVC (uPVC) is used in rigid plumbing pipes and window frames.
• Plasticised PVC is used in flexible electrical cable insulation, hoses, and flooring.

Both contain the same basic PVC repeat unit:

\[ \ce{[-CH2-CH(Cl)-]_{n}} \]

The difference is the formulation.

Unplasticised PVC (uPVC)

Unplasticised PVC does not contain the plasticisers used to make flexible PVC.

Its polymer chains have limited mobility under ordinary conditions, so the material remains relatively rigid.

This makes it useful for products that need to retain their shape, such as plumbing pipes and window frames.

Plasticised PVC

Plasticisers are substances added to PVC to increase its flexibility.

They sit between polymer chains and make it easier for the chains to move relative to one another.

As a result, the finished material becomes softer and more flexible.

Here is the important point:

Adding a plasticiser does not change the PVC repeat unit.

The polymer still contains the same carbon backbone and chlorine substituents. The plasticiser is an additional substance in the material, not a replacement for the original monomer and not a new repeating unit.

This is why you cannot identify PVC simply by testing whether a plastic object is rigid or flexible.

05Comparing their uses and properties

Now that we understand the structural differences, we can connect them to real applications.

Does the application tell you which polymer was used?

Sometimes it gives you a useful clue, but it is not definitive.

For example, a flexible cable sheath could be plasticised PVC. Its flexibility does not prove that it is polyethylene.

Likewise, a rigid water pipe could be HDPE rather than PVC.

The material’s intended use depends on more than its repeat unit. Manufacturers also consider the polymer grade, additives, processing conditions, and the performance required from the finished product.

For HSC Chemistry questions, focus on explaining the connection between structure and properties rather than memorising a list of products.

06How to approach PVC and polyethylene structure questions

When you are given a monomer or polymer structure, use this approach.

Step 1: Identify the monomer.

Look for the carbon-carbon double bond and identify the atoms attached to each carbon.

For PVC, the monomer is chloroethene:

\[ \ce{CH2=CHCl} \]

For polyethylene, the monomer is ethene:

\[ \ce{CH2=CH2} \]

Step 2: Identify the carbon backbone.

In addition polymerisation, the double bond opens and the monomers connect through carbon-carbon single bonds.

Step 3: Preserve the substituents.

Do not remove the chlorine atom when drawing PVC.

It remains attached to the same carbon atom in the repeat unit.

Step 4: Connect the structure to the property.

Consider polarity, intermolecular attractions, branching, and additives.

For example:

• Polar C-Cl bonds contribute to intermolecular attractions in PVC.
• Branching in LDPE reduces how closely polymer chains can pack.
• Plasticisers increase the mobility of PVC chains.

One final distinction: ethene is the monomer for polyethylene, but it is not the monomer for PVC. Ethene must first be converted into chloroethene before PVC can be manufactured.

If you want to understand that industrial pathway, see Fractional Distillation vs Cracking: From Crude Oil to PVC.

07Worked examples

08Questions and solutions

Question 1

A polymer has the following repeating structure:

\[ \ce{[-CH2-CH(Cl)-]_{n}} \]

Identify:

a) The polymer’s common name.

b) Its monomer.

c) The molecular formula of its repeat unit.

Solution 1

The polymer is PVC, and its monomer is chloroethene, \(\ce{CH2=CHCl}\).

The repeat unit has the molecular formula \(\ce{C2H3Cl}\).

The structure contains a two-carbon backbone with a chlorine substituent attached to every second carbon.

The monomer must contain a carbon-carbon double bond so that addition polymerisation can form the continuous carbon backbone.

A common mistake is to write ethene as the monomer. Ethene forms polyethylene, not PVC. The chlorine atom must already be present in the chloroethene monomer.

Question 2

A student compares polyethylene and PVC and makes the following statement:

“PVC contains chlorine atoms that form covalent bonds between neighbouring polymer chains. This is why PVC is more rigid than polyethylene.”

Evaluate this statement.

In your answer, discuss:

a) The bonding within the PVC repeat unit.

b) The intermolecular attractions between neighbouring PVC chains.

c) How these attractions compare with those in polyethylene.

Solution 2

The statement is incorrect because chlorine atoms are covalently bonded within individual PVC chains, not between neighbouring polymer chains.

The chlorine atom is attached to a carbon atom in the PVC backbone through a polar covalent C-Cl bond.

This creates permanent dipoles within the polymer structure. Neighbouring PVC chains can experience dipole-dipole attractions as well as London dispersion forces.

Polyethylene is largely non-polar and experiences mainly London dispersion forces between its chains.

These differences help explain the distinct physical properties of the polymers.

However, it would be too simplistic to conclude that every PVC product must be rigid. Plasticisers can increase the mobility of PVC chains, producing flexible materials without changing the polymer’s repeat unit.

The important distinction is between covalent bonds within a polymer chain and intermolecular attractions between separate polymer chains.

Question 3

Two polymer samples have the same average molar mass of \(1.40\times10^5\text{ g mol}^{-1}\) per polymer chain.

One sample is polyethylene, and the other is PVC.

Calculate the approximate number of repeat units in an average chain of each polymer.

Use:

• PE repeat unit molar mass = \(28.0\text{ g mol}^{-1}\).
• PVC repeat unit molar mass = \(62.5\text{ g mol}^{-1}\).

Assume that chain-end effects are negligible.

Solution 3

The average polyethylene chain contains approximately 5000 repeat units, while the average PVC chain contains approximately 2240 repeat units.

To calculate the number of repeat units, divide the average polymer chain molar mass by the molar mass of one repeat unit.

Step 1: Calculate the number of polyethylene repeat units.

\[ \begin{aligned} N_{\text{PE}} &=\frac{1.40\times10^5}{28.0}\\ &=5000 \end{aligned} \]

Step 2: Calculate the number of PVC repeat units.

\[ \begin{aligned} N_{\text{PVC}} &=\frac{1.40\times10^5}{62.5}\\ &=2240 \end{aligned} \]

Step 3: Interpret the results.

Both chains have the same average molar mass, but the polyethylene chain contains more repeat units.

This happens because each polyethylene repeat unit has a lower molar mass than each PVC repeat unit.

The result does not mean that polyethylene has stronger bonds or that its polymer chains must be physically longer. It tells us how many repeating sections are present in each average chain, under the stated assumptions.

Question 4

A manufacturer produces two grades of polyethylene.

The following information is obtained:

Both samples have the repeat unit:

\[ \ce{[-CH2-CH2-]_{n}} \]

a) Identify which sample is LDPE and which is HDPE.

b) Explain why their densities differ.

c) Explain why their physical properties differ despite having the same repeat unit.

d) Does the difference mean that the two samples are made from different monomers? Justify your answer.

Solution 4

Sample A is LDPE, and Sample B is HDPE. Both are made from ethene and have the same basic repeat unit.

a) Identify the polymers.

Sample A is LDPE because it contains more branching and has a lower density.

Sample B is HDPE because its chains are more linear and can pack more closely.

b) Explain the density difference.

The branches in LDPE interfere with close packing between neighbouring polymer chains.

This reduces the degree of crystallinity and generally produces a lower-density material.

HDPE has fewer branches, allowing its chains to pack more closely together. This produces a higher-density material.

c) Explain the difference in physical properties.

The closer packing in HDPE generally produces greater stiffness and rigidity.

The more branched structure of LDPE prevents such close packing, resulting in a more flexible material.

Therefore, differences in chain architecture influence the physical properties even when the repeating chemical structure is identical.

d) Identify whether the monomers differ.

No. Both polymers are made from ethene, \(\ce{CH2=CH2}\).

Their repeat units are also identical.

The difference is the arrangement of the polymer chains, particularly the extent of branching.

This is an important distinction: the chemical identity of a repeat unit does not completely determine the properties of a finished polymer.

Question 5

A plastics manufacturer receives two batches of polymer resin, each with a mass of 250.0 g.

The laboratory conducts an elemental analysis and obtains the following results.

Both materials are addition homopolymers formed from two-carbon alkene monomers.

Further analysis establishes that:

• Batch A is used to manufacture flexible electrical cable insulation. Its formulation includes a plasticiser.
• Batch B is used to manufacture rigid water pipes. Its polymer chains are mostly linear, with relatively few carbon-chain branches.

A supplier makes three claims:

1. Batch A must be polyethylene because it is flexible.
2. Batch B must be PVC because it is rigid.
3. Adding a plasticiser to Batch B would turn it into LDPE.

Evaluate all three claims.

Your response must:

a) Determine the empirical formula of the repeat unit in each batch using the elemental analysis.

b) Identify each polymer and its monomer.

c) Calculate the amount of repeat units in each 250.0 g sample.

d) Explain the physical behaviour of both materials.

e) Explain whether adding or removing a plasticiser changes the identity of the polymer or its chain architecture.

Use atomic masses C = 12.0, H = 1.0, and Cl = 35.5. Assume the samples are pure polymer resins, contain no other elements, and have negligible chain-end effects. All chlorine in Batch A belongs to its polymer repeat units.

Solution 5

All three supplier claims are incorrect.

Batch A is PVC, despite being flexible, because its formulation contains a plasticiser. Batch B is polyethylene, despite being rigid, because its mostly linear chains allow close packing.

Adding a plasticiser cannot turn a linear polyethylene structure into the branched chain architecture characteristic of LDPE.

Let’s establish each conclusion using the chemical evidence.

Step 1: Determine the repeat unit in Batch A.

Convert each elemental mass into an amount in moles of atoms.

For carbon:

\[ n(\mathrm C)=\frac{96.0}{12.0}=8.00\text{ mol} \]

For hydrogen:

\[ n(\mathrm H)=\frac{12.0}{1.0}=12.0\text{ mol} \]

For chlorine:

\[ n(\mathrm{Cl})=\frac{142.0}{35.5}=4.00\text{ mol} \]

The mole ratio is:

\[ 8.00:12.0:4.00 \]

Divide each value by the smallest number, 4.00:

\[ 2:3:1 \]

Therefore, the empirical formula is:

\[ \ce{C2H3Cl} \]

This matches the PVC repeat unit.

Step 2: Determine the repeat unit in Batch B.

For carbon:

\[ n(\mathrm C)=\frac{214.3}{12.0} =17.86\text{ mol} \]

For hydrogen:

\[ n(\mathrm H)=\frac{35.7}{1.0} =35.7\text{ mol} \]

There is no chlorine.

The carbon-to-hydrogen mole ratio is approximately:

\[ 17.86:35.7 \]

Divide by 17.86:

\[ 1:2 \]

Expressed using two carbon atoms, the empirical formula is:

\[ \ce{C2H4} \]

This matches the polyethylene repeat unit.

Step 3: Identify the monomers and polymers.

Batch A contains PVC.

Its repeat unit is:

\[ \ce{[-CH2-CH(Cl)-]} \]

Its monomer is chloroethene:

\[ \ce{CH2=CHCl} \]

Batch B contains polyethylene.

Its repeat unit is:

\[ \ce{[-CH2-CH2-]} \]

Its monomer is ethene:

\[ \ce{CH2=CH2} \]

Step 4: Calculate the amount of repeat units in each batch.

First, calculate the molar masses.

For PVC:

\[ M_{\text{PVC}}=62.5\text{ g mol}^{-1} \]

For polyethylene:

\[ M_{\text{PE}}=28.0\text{ g mol}^{-1} \]

Use:

\[ n=\frac{m}{M} \]

For Batch A:

\[ n_{\text{PVC}}=\frac{250.0}{62.5} =4.00\text{ mol} \]

For Batch B:

\[ n_{\text{PE}}=\frac{250.0}{28.0} =8.93\text{ mol} \]

Therefore, Batch A contains 4.00 mol of repeat units, while Batch B contains approximately 8.93 mol.

The polyethylene batch contains more repeat units because each repeat unit has a lower molar mass.

Step 5: Explain why the supplier’s first two claims are wrong.

The supplier assumes that flexibility identifies polyethylene and rigidity identifies PVC.

That assumption fails because the physical behaviour of a polymer depends on more than its repeat unit.

Batch A is PVC, but the plasticiser increases the mobility of its polymer chains, making the finished material flexible.

Batch B is polyethylene. Its mostly linear chains pack relatively closely together, producing a higher-density, more rigid material.

The elemental analysis provides much stronger evidence of polymer identity than the physical appearance of the products.

Step 6: Evaluate the effect of adding or removing plasticiser.

Removing the plasticiser from Batch A would leave the PVC repeat unit unchanged.

The material would generally become less flexible because the polymer chains would have less freedom to move relative to one another.

However, it would still be PVC. Its monomer and polymer backbone would not change.

Similarly, adding an additive to Batch B might change some of its physical properties, depending on the formulation. But it would not create the carbon-chain branches characteristic of LDPE.

LDPE and HDPE differ in their polymer chain architecture, not simply in how much plasticiser they contain.

The key lesson is that three different levels of structure must be kept separate:

• Repeat unit: Determines the chemical identity of the polymer, such as PVC or polyethylene.
• Chain architecture: Determines features such as branching, as seen in LDPE and HDPE.
• Formulation: Includes additives such as plasticisers that modify the properties of the finished material.

Confusing these three levels is exactly what leads to the supplier’s incorrect conclusions.

09Where to go next

The PVC versus polyethylene comparison is a useful starting point for a broader idea in polymer chemistry: a material’s properties depend on both its chemical structure and how its chains are arranged.

Once you can identify the monomer and repeat unit, the next step is to consider how those structures influence physical properties, and how additives change the behaviour of the finished product.

For a separate examination of monomer hazards, polymer properties, additives, and environmental impacts, read Monomers vs Polymers: Health and Environmental Impacts.